Limited efficiency of wet scrubbers in reducing the environmental impact of ship-emitted particles

Sulfur dioxide pollution by ship emissions can be efficiently decreased by using exhaust gas scrubbers, yet particles can pass through the scrubber and be released into the atmosphere. Here, we studied the impact of using a wet scrubber on the composition of particle emissions, by single-particle analysis. At low engine loads, results show no significant changes in particle composition of metals, salts, and polycyclic aromatic hydrocarbons (PAH). At high engine loads, the scrubber reduced soot and PAH signatures about fourfold. Particles passing through the scrubber undergo minimal chemical changes, except for sulfate uptake. The cleaning effect of wet scrubbers is attributed to the removal of water-soluble gas-phase compounds, diffusion-dominated uptake of ultrafine particles, and wet deposition of coarse particles. The scrubber has little effect on reducing the health and environmental impacts of the remaining particles that pass through it. These emitted particles, primarily in the 60–200 nm size range, constitute a significant portion of the inhalable particle mass and have the potential for long-range transport.